Modern day locomotive controls, including the locomotive brake control system, incorporate computer technology to reduce hardware and to facilitate adaption of the system to various customer requirements.
In one such brake control system, a cab mounted, handle operated, brake command controller outputs the desired brake command signal to a microprocessor unit, which interprets the brake command signal in terms of a feedback signal indicative of the air pressure in an equalizer reservoir. The microprocessor then effects the operation of either the application or the release electro-pneumatic valve to adjust the equalizer reservoir pressure in accordance with the brake command signal.
Responsive to the aforementioned brake control circuit, as illustrated in FIG. 1, a high capacity pneumatic relay valve device is employed to vary the trainline brake pipe pressure in accordance with variations of the pressure present in the equalizing reservoir, in order to control the railway car brakes. The aforementioned subsystem is called a brake pipe control circuit and its application to a locomotive brake control system is shown and described in the above-cited references, previously incorporated herein by reference.
The brake control system further includes an automatic brake control circuit, as illustrated in FIG. 1, having each of electro-pneumatic application and release valves, as well as an independent brake control circuit, illustrated also in FIG. 1, which includes additional electro-pneumatic application and release valves. The automatic brake control system is primarily intended for use when a train consist is made-up, for applying the brakes to all cars as well as the locomotives therein.
In contrast thereto, the independent brake control system is primarily intended for use when one or more locomotives are driven independently of any cars connected therewith when the automatic brake control electro-pneumatic valves are operated by the microprocessor in response to changes in brake pipe pressure initiated by the brake pipe control circuit in accordance with movement of an automatic brake handle of the brake command controller. There is another high capacity pneumatic relay valve device which regulates the pressure in the locomotive brake cylinders according to the pressure output.
The electro-pneumatic valves in the brake pipe control circuit and in the locomotive brake cylinder control circuit are arranged to assume a pressure release state, in the event of a power loss at the microprocessor unit. In consequence of such a power loss, brake pipe pressure is reduced while, concurrently, the locomotive brake cylinder pressure is released.
A pneumatic back-up control valve in the locomotive automatic brake control circuit is provided to establish locomotive brake cylinder pressure in response to the aforementioned reduction of brake pipe pressure resulting from such fail-safe operation of the electro-pneumatic valves in the brake pipe control circuit, there being a double check valve in order to separate the pneumatic backup control valve from such electro-pneumatic valves in the locomotive brake cylinder control circuit.
The pneumatic backup control valve includes a piston valve assembly, subject on opposite sides to compressed air in the brake pipe and in the control reservoir. When brake pipe pressure is reduced, the resultant pressure differential forces the piston valve assembly to an application position wherein the control reservoir air supplies the brake cylinder pilot line to establish the locomotive brake pressure until a force balance is restored across the piston valve assembly. In this manner, the piston valve assembly seeks a lap position in which the supply of brake cylinder pressure is terminated at a value corresponding to the brake pipe pressure reduction in effect.
The locomotive brake cylinder pressure may be released independently of the car brakes by means of a quick release valve associated with the pneumatic backup control valve. A pressure signal supplied to the quick release valve, when a quick release switch is activated, initiates this "bail-off" or quick release function. The brake pipe and control reservoir pressures are communicated via the quick release valve to establish pressure equalization across the piston valve assembly, when the quick release switch is actuated. In this manner, the control reservoir pressure is effectively equalized with the reduced brake pipe pressure, such that a spring force acting on the piston valve assembly is effective to force the piston valve assembly to release position and the locomotive brake cylinder pressure is exhausted.
In order to reapply the brakes, the quick release switch is deactivated to interrupt communication between the brake pipe and control reservoir pressures via the quick release valve, so that a subsequent reduction of brake pipe pressure is effective to produce a further pressure differential across the piston valve assembly. In response to this further pressure differential, the piston valve assembly will return to the application position and reestablish the supply of control reservoir pressure to the brake cylinder pilot line to reapply the locomotive brake.
In the event of a power failure at the microprocessor, brake pipe pressure is reduced to zero due to the fail-safe configuration of the electro-pneumatic valves in the brake pipe control circuit which would, therefore, allow the locomotive to be moved without the ability to obtain any automatic locomotive braking whatsoever. The aforementioned U.S. Pat. No. 5,222,788 discloses a circuit modification which provides correction of this shortcoming by providing a microprocessor having a regulated source of electrical power for the control of such first electro-pneumatic valve in accordance with the brake command signal. Additionally, there is a brake assurance arrangement provided for establishing the energized condition of the second electro-pneumatic valve in response to the loss of the regulated source of electrical power.
The aforementioned braking control systems have been provided to address the situations in which a power failure has inactivated such electro-pneumatic valves, in particular those used in the brake control circuit for the independent brake control system illustrated in FIG. 1.
U.S. Pat. No. 5,503,469 addresses the situation in which the brake pipe control system and the automatic brake control system have been deliberately deactivated based upon the arrangement of the train consist and an electrical failure occurs in the independent brake control system. This system keeps the critical independent braking system operative despite electrical failures affecting the electro-pneumatic valves.
The independent braking system is particularly important since the pressure in control reservoir is critical to the activation of relay valve which in turn regulates pressure to the independent application release brake line. Pressure on this brake line is used to develop the control signals of the automatic brake control system, including control reservoir which is used to trigger relay valve and thereby control the main brake cylinder. Consequently, any failure in any of the aforementioned elements could prove critical to the operability of the train braking system, especially under the circumstances described in the aforementioned braking systems.
While techniques have been developed for failures in the brake control systems, adequate solutions for the failure of a major relay valve have not been provided in this art.